The present invention relates in general to external-cavity semiconductor lasers. It relates in particular to optically-pumped, intracavity (IC) frequency-doubled, semiconductor lasers.
Intracavity doubled semiconductor lasers are known in the prior art in two classes. One class is based on edge-emitting semiconductor lasers (diode-lasers), the other on vertical-cavity surface-emitting lasers (VCSEL), electrically-energized. In each class, in order to conveniently effect intracavity doubling, the resonant-cavity of the semiconductor laser must be extended beyond the semiconductor device, leaving free space in which an optically-nonlinear crystal can be located to effect frequency-doubling. This is usually effected by providing an antireflection coating on the emitting surface of the semiconductor laser (which would otherwise serve as an output coupling mirror) and by providing an external-mirror, spaced apart from that surface, to serve the same purpose. Such an arrangement is usually referred to as an external-cavity semiconductor laser.
The efficiency of frequency-conversion in an optically-nonlinear crystal is directly proportional to power (intensity) of the fundamental frequency incident on the crystal. This is the case whether conversion is doubling to a second harmonic frequency, frequency mixing to provide third or higher odd harmonic frequencies, or optical parametric oscillation (OPO). Because of this, for example, for a useful IC-doubling, a high power (about 200 milliwatts (mW) or greater) semiconductor laser is essentially a prerequisite. Unfortunately, in both classes of semiconductor laser which have been used in the prior-art for this purpose, increasing power comes at the expense of decreasing beam-quality.
An edge-emitting semiconductor laser, for example, is inherently a high-gain device, as laser light resonates in the plane of the layers forming its active or gain region. As the height (thickness) of these gain-region layers is constrained by electrical confinement and optical confinement considerations, output power must be increased by increasing the width of the gain-region. As the width of the gain-region is increased (to as much as one-hundred times its height in high-power devices), more modes can oscillate, and the output beam becomes highly astigmatic. Accordingly, design of an adequate resonator, for coupling light into an optically-nonlinear crystal therein, as well as for general beam-quality, becomes increasingly more difficult, if not impossible.
A VCSEL is inherently a relatively low gain device, as laser-radiation resonates perpendicular to the plane of the layers forming its active or gain-region. For a relatively small beam diameter, for example about 5 micrometer (xcexcm) or less, single-mode operation and high beam-quality can be achieved. Gain and output power can be improved in part by increasing the number of active layers in the gain medium. This is somewhat limited by considerations of the properties of materials forming the semiconductor structure. For a further increase in power, however, the area of the emitting surface must be increased. Increasing the emitting surface area to a diameter greater than about 5 xcexcm inevitably leads, initially, to multimode operation. Further increasing the diameter leads to problems in energizing the laser, as electrical pumping must be supplied laterally. In order to do this uniformly and efficiently, the electrical resistance of semiconductor layers forming the laser must be decreased by increased doping. Increased doping, however, reduces the light transmission of the layers and increases resonator loss, such that the purpose of increased doping quickly becomes self-defeating.
There is a need for an intracavity frequency-converted external-cavity semiconductor laser that can provide high, frequency-converted output power together with high beam-quality.
The general structure of this type of laser is set forth in U.S. patent application Ser. No. 09/179,022, filed Oct. 26, 1998, the disclosure of which is incorporated herein by reference. The subject invention represents a specific commercial embodiment of such a device.
Additional information about designing and fabricating high power lasers of this type can be found in U.S. patent application Ser. No. 09/263,325, filed Mar. 5, 1999, the disclosure of which is herein incorporated by reference.
In one aspect, a laser in accordance with the present invention comprises an OPS-structure including a multilayer gain-structure surmounting a multilayer mirror-structure. The gain-structure includes a plurality of active layers spaced apart by spacer layers. A laser-resonator is formed between the mirror structure of the OPS-structure and an external mirror spaced apart from the OPS-structure. A pump-light source is arranged to deliver pump-light to the gain-structure for generating laser-radiation in the laser-resonator. A transmissive wavelength-selective element is located in the laser-resonator for selecting a frequency of the laser-radiation within a gain-bandwidth characteristic of the composition of the gain-structure. An optically-nonlinear crystal is located in the laser-resonator between the transmissive wavelength-selective element and the optically-nonlinear crystal and is arranged to double the selected frequency of laser-radiation, thereby providing frequency-doubled radiation. A first dichroic filter is disposed between the gain-structure and the optically-nonlinear crystal. The first dichroic filter is at normal incidence to the resonator axis and is arranged to reflect frequency-doubled radiation incident thereon and direct the frequency-doubled radiation toward a second dichroic filter. The second dichroic filter is located between the transmissive wavelength-selective element and the optically-nonlinear crystal, and is inclined at an angle to the resonator axis. The second dichroic filter is arranged to reflect frequency-doubled radiation directed thereto by the first dichroic filter out of the laser-resonator as output radiation.
The transmissive wavelength-selective element is preferably a birefringent filter. Use of another transmissive wavelength-selective element such as an etalon, however, is not precluded.
Preferably, the first dichroic filter is in the form of a coating on the gain-structure and the second dichroic filter is in the form of a coating on a surface of the transmissive wavelength-selective element facing the optically-nonlinear crystal. This arrangement of dichroic filters avoids resonator losses due to polarization rotation of fundamental laser-radiation which could occur were the filters coated on separate substrates. This arrangement also provides that frequency-doubled radiation does not pass through any optical elements other than the optically-nonlinear crystal in which the frequency-doubled radiation is generated. This maximizes the output of frequency-doubled radiation.
In one example of the inventive laser having this particular arrangement of dichroic filters, the gain-structure has active layers of indium gallium arsenide with a composition selected to provide laser-radiation at a wavelength of about 976 nanometers (nm). The gain-structure is optically pumped by 2 Watts (W) of focussed light from an edge-emitting diode-laser, the pump-light having a wavelength of about 808 nm. The birefringent filter is a crystal quartz plate inclined at about 57xc2x0 to the resonator axis. The optically nonlinear crystal is crystal of LBO (lithium tri-borate LiB3O5). The external mirror is a concave mirror having a radius of curvature of 5.0 millimeters (mm) spaced apart by a distance of about 4.3 mm from the OPS-structure. The laser delivers between about 20 and 60 milliwatts (mW) of frequency-doubled (488 nm) radiation in a single axial mode.